1. Field of the Invention
The present invention relates to a waveguide optical device, and, in particular, to an array waveguide wavelength mixing/branching device which mixes light with different wavelengths (mixing) and/or decomposes multiplexed light for respective wavelengths (branching).
2. Description of the Related Art
In order to achieve efficient and positive signal transmission, in a technical field such as a wavelength multiplex optical telecommunications field or an optical signal processing field, an improvement in performance of waveguide integrated optical device, in particular, waveguide optical devices such as array waveguide wavelength mixing/branching device is demanded.
In a wavelength multiplexing optical telecommunications in which simultaneously a plurality of optical signals having different wavelengths are transmitted, a wavelength mixing/branching device which mixes or braches optical signals having different wavelengths is an important device. Especially, a wavelength mixing/branching device (referred to as an array waveguide wavelength mixing/branching device, hereinafter) using an array waveguide diffraction grating which includes an optical waveguide elements formed on a plane substrate attracts attention as a practical device in the above-mentioned telecommunications system.
FIG. 1 shows a plan view of such an array waveguide wavelength mixing/branching device in the related art. As shown in the figure, this mixing/branching device has a configuration including an input waveguide 1, an input slab waveguide 2, a channel waveguide 3, an output slab waveguide 4, and an output waveguide 5 connected in sequence formed on a substrate 100.
After the optical path of an optical signal incident into the input waveguide 1 from the outside via an optical fiber is expanded through diffraction in the input slab waveguide 2, the optical signal is incident into the plurality of waveguide elements of the channel waveguide 3. This incident optical signal arrives at the output slab waveguide 4 after propagating through the channel waveguide 3. At this time, the optical signal radiating from the plurality of waveguide elements 3 as a form of a plurality of light elements interferes with each other of the respective light elements, condenses near the connection point between the output slab waveguide 4 and the output waveguide 5, then is incident into respective waveguide elements of the output waveguide 5. After that, these light elements of the optical signal are led to the substrate end.
Due to phase difference occurring due to the difference in optical propagation distance between the plurality of waveguide elements of the channel waveguide 3, the positions at which the respective light elements of the optical signal condense differ from each other according to the wavelengths thereof. Thanks to this effect, the light elements of the optical signal having different wavelengths are obtained from the respective waveguide elements of the output waveguide 5. Thus, the wavelength-multiplexed optical signal is demultiplexed into the optical signals having the respective wavelengths. According to the same principle, this type of AWG can also be used for combining/multiplexing the plurality optical signals having different wavelengths into the wavelength-multiplexed optical signal.
The above-described array waveguide wavelength mixing/branching device may be produced as the waveguide elements are together formed on the plane substrate through a photolithography technique. Then, as described above, the diffraction grating thus produced is utilized to provide a function same as a spectrometer. Thus, the array waveguide wavelength mixing/branching device is an effectively miniaturized optical device, can be produced in a mass-production manner and, as a result, especially attracts attention as a suitable wavelength mixing/branching device for the field of wavelength multiplexing telecommunications. Such array waveguide wavelength mixing/branching device may be simply referred to as an AWG (Arrayed Waveguide Grating), hereinafter.
Such an AWG includes the slab waveguide and the channel waveguide as mentioned above. There, as shown in FIG. 1, cores 91 having an medium refractive index are formed in a base 90 having a low refractive index, such a configuration is called xe2x80x98medium-refractive-index type AWGxe2x80x99, and will now be referred to as a first related art. According to the first related art, the chip size may become larger, and, thus, miniaturization and cost reduction may not be achieved sufficiently, as the bending radius of the channel waveguide 3 cannot be made shaper efficiently because the refractive index thereof is not so high.
In order to realize the miniaturization of the chip, the channel waveguide needs to be bent much and thus, it is necessary to make the bending radius smaller. For this purpose, as shown in FIG. 2, cores 91 of a high refractive index is formed in a base 90 of a low refractive index. Namely, the ratio xcex94n in refractive index between the low refractive index part (base) and the high refractive index part (cores) is made larger. Such a configuration (high-refractive-index type AWG) will now be referred to as a second related art. Specifically, this ratio xcex94n is made as high as 1.5% as a high specific refractive index. Thereby, the bending radius of the channel waveguide can be reduced into 2 mm. In contrast thereto, in a case (for example, the case shown in FIG. 1) where this ratio xcex94n is 0.5% (low specific refractive index) the bending radius is 20 mm. Accordingly, it is possible to remarkably reduce the size of the chip by employing the manner described above with reference to FIG. 2.
However, when enlarging this ratio xcex94n, the core size should also be reduced at the same time in order to fulfill a so-called single mode waveguide requirement of signal light. Thereby, a problem occurs in that optical coupling loss becomes larger at a connection with a single mode optical fiber. Specifically, in the case of xcex94n being 1.5%, the coupling loss becomes 2.1 dB which is remarkably higher than 0.1 dB in the case where xcex94n is 0.5%.
For solution of this problem, as shown in FIGS. 3 and 4, a mode-transformation part 11 is provided in a form of taper shape or the like. Then, therethrough, in a midway of the input waveguide 1, a part (medium-refractive-index core 91) directly connected with an optical fiber having a refractive index of 0.5% is connected with a part directly connected with the input slab 2 having a refractive index of 1.5%. Similarly, through a mode transformation part 55, in a midway of the output waveguide 5, a part (medium-refractive-index cores directly connected with an optical fiber having a refractive index of 0.5% is connected with a part directly connected with the output slab 4 having a refractive index of 1.5%. Thereby, it becomes possible to achieve reduced loss coupling with the optical fibers, and at the same time, to achieve the miniaturization of AWG. Such a technology (double-core high-refractive-index type AWG) will be refereed to as a third related art, hereinafter.
However, in the AWG according to the third related art described above with reference to FIGS. 3 and 4, mode transformation loss occurs in the mode transformation parts 11 and 55. Thereby, sufficient loss reduction of AWG cannot be achieved. Moreover, in the manufacturing process of waveguides on the AWG, high precision is required for photomask alignment in the mode transformation part between the pattern of xcex94n=0.5% and the pattern of xcex94n=1.5%. Then, if the photomask alignment accuracy is degraded into the order of 0.1 micrometers, extra loss occurring in the mode transformation part becomes more than 1 dB. This matter also causes difficulty in loss reduction of AWG.
An object of the present invention is to provide a waveguide optical device in which such a mode transformation loss as that occurring in the third related art can be eliminated so as to effectively reduce the optical propagation loss, and also, manufacture thereof is easier.
According to the present invention, a channel core pattern includes a plurality of core pattern elements having different lengths (channel waveguide 3), and a refractive index of the channel core pattern there is higher than a refractive index of another core pattern (to be connected to an external optical fiber). As the core pattern elements of the channel core pattern has the relatively high refractive index, sharp bending of the channel core pattern can be allowed. Also, as the another core pattern having the relatively low refractive index is used for connecting with the external optical fiber, the degree of mode transformation to be made in the connection with the optical fiber can be effectively reduced, and thus, the mode transformation loss can be reduced.
Furthermore, according to the present invention, connection between the channel core having the relatively high refractive index and the fiber connecting core having the relatively low refractive index can be made in a zone at which the core width thereof is wider.
Accordingly, it is possible to achieve miniaturize and cost reduction of the waveguide optical device, and, also, to effectively reduce the optical propagation loss.
Furthermore, as the connection between the core part having the relatively high refractive index and the core part having the relatively low refractive index is made at the zone at which the core width thereof is wider, it is possible to remarkably ease the management of photomask alignment error at the time of manufacture, and, as a result, to reduce the manufacture cost.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.